As is well known in the prior art, the absorption of photons induces an excitation of molecular energy levels, which in turn may lead to a change of the sample temperature, pressure and density. Changes of these properties may be used for trace detection in photothermal spectroscopy (PTS). The techniques use laser radiation to generate transient changes of the sample properties; if the temperature rise caused by photo-absorption is fast enough a pressure change within the sample is generated, which will disperse in an acoustic wave. Once the pressure has relaxed to the equilibrium pressure, a density change proportional to the temperature will remain. In combination, temperature and density changes affect other properties of the sample, such as the refractive index. PTS methods are based on changes in the sample temperature, typically monitored trough the refractive index of the sample. In contrast to classical transmission spectroscopy according to Lambert-Beer law where sensitivity is increased with path length, PTS is an indirect method for optical absorption analysis, which measures a photo-induced change in the thermal state of the sample. For this reason, PTS offers the possibility for sensor miniaturization. PTS signals are generally proportional to the temperature change and inversely proportional to the excitation volume. The latter arises because higher temperature changes can be induced in smaller volumes with a given power, and also because PT signals may be derived from a spatial gradient in the resulting refractive index change. The deposited heat power is proportional to the absorption coefficient of the sample and the incident light intensity.
PTS setups detecting refractive index changes typically employ an excitation laser source for sample heating and a probe laser source to monitor changes resulting from heating. The change in the refractive index cause a phase shift of light passing through the heated sample which can be measured with high sensitivity using an interferometer.
It was already proposed in the art to use Fabry-Perot interferometers (FPI) for photothermal trace gas detection, see for example A. J. Campillo, S. J. Petuchowski, C. C. Davis, and H.-B. Lin “Fabry-Perot photothermal trace detection”, Appl. Phys. Lett. 41(4), 327-329 (1982) or B. C. Yip, and E. S. Yeung, “Wavelength modulated Fabry-Perot Interferometry for quantitation of trace gas componentes,” Anal. Chim. Acta 169, 385-389 (1985).
Fabry-Perot interferometers use an optical cavity for multi-wave interference instead of a single-pass interferometer design. The FPI comprises two parallel partially reflecting mirrors within a beam undergoes multiple reflections. Refractive index changes can be measured comparatively easily by measuring the transmitted light intensity through the FPI, which is dependent on the phase shift of the light.
Such design was also proposed in Yip, “Trace detection in gases using photoacoustic spectroscopy and Fabry-Perot interferometry” (1984). In a dual-beam arrangement, two parallel optical paths are introduced into the interferometer by splitting the output of a single-frequency laser, one for the sample interaction chamber which contains the species of interest, the other for a reference chamber containing the buffer gas only. In this way, residual background absorption can be accounted for when both the reference and sample chambers are irradiated with the excitation beam.
Thus, in distinction from the invention the arrangement of Yip provides for the irradiation of both chambers with the excitation beam and furthermore that merely the sample interaction chamber, not the reference chamber contains the species of interest.
However, experimental results from the applicants suggest that the known arrangement of Yip is incapable of sufficiently eliminating acoustic and thermal noise as well as compensating for changes in the composition of the matrix.
The article Yang et. al., “Hollow-core fiber Fabry-Perot photothermal gas sensor”, Optics Letters, Vol. 41, No. 13 discloses a trace gas sensor based on photothermal effect in a hollow-core fiber Fabry-Perot interferometer. A reference gas cell is used to estimate the gas concentration by directly measuring the attenuation of the transmitted light.
However, the photothermal interferometry setups proposed in the prior art entailed a number of drawbacks. First, the transmission signal was prone to probe laser phase noise, probe laser intensity noise, acoustic noise and mechanical noise. Second, the known setups lacked stability so that use was essentially restricted to laboratory environments. For example, in the prior art moveable parts, such as piezo elements for adjusting the distance between the FPI cavity mirrors, were used to tune the transmission of the probe laser radiation through the FPI cavity. Third, the selectivity and sensitivity was insufficient in certain applications. Fourth, considering gas sensing in a complex gas matrix with varying composition and/or varying temperature the refractive index of the matrix is varying, too. This in turn will lead to a change in the measured probe laser intensity independent of the analyte concentration. Thus, to allow for optimal and constant coupling to the cavity it is suggested to adjust the cavity length. This adds to complexity and instability of the overall measurement system.
It is an object to alleviate or eliminate at least one of the drawbacks of the prior art.
According to an aspect of the invention, a photothermal interferometry apparatus for detecting a molecule in a sample, in particular for detecting a trace gas species, comprises:
a Fabry-Perot interferometer with a first mirror, a second mirror and a first cavity for containing the sample extending between the first and the second mirror,
a probe laser arrangement with at least one probe laser for providing a first probe laser beam and a second probe laser beam,
an excitation laser for passing an excitation laser beam through the first cavity of the Fabry-Perot interferometer for exciting the molecule in the sample,
the Fabry-Perot interferometer comprising a third mirror, a fourth mirror and a second cavity for containing the sample extending between the third and the fourth mirror,
the first and the second cavity of the Fabry-Perot interferometer being arranged such that the first probe laser beam intersects with the excitation laser beam in the first cavity and the second probe laser beam does not intersect with the excitation laser beam in the second cavity,
a photodetector unit comprising a first photo detector for detecting the transmitted first probe laser beam and a second photo detector for detecting the transmitted second probe laser beam.
In a preferred embodiment, the probe laser arrangement comprises a beam splitter for splitting a probe laser beam from the probe laser into the first and second probe laser beam. Thus, in this embodiment, the probe laser beam is split into a first and a second probe laser beam before being passed through the first and second cavity of the Fabry-Perot interferometer, respectively. Alternatively, the probe laser arrangement comprises two probe lasers, the first probe laser being arranged for providing the first probe laser beam for the first cavity and the second probe laser being arranged for providing the second probe laser beam for the second cavity.
In the first cavity, the first probe laser beam passes through the sample (containing the molecule of interest, i.e. the analyte) heated by the excitation laser beam such that the transmission of the first probe laser beam detected in the first photo detector is influenced by the heating of the sample with the excitation laser beam. In the second cavity, the second probe laser beam passes through the same sample but unaffected by heating with the excitation laser beam. The second photo detector receives the transmission of the second probe laser beam. In this way, the first probe laser beam probes both noise, in particular probe laser phase noise and surrounding noise (mechanical and acoustical noise) as well as noise due to changing matrix composition, and the photothermal phase shift spectroscopy (PTPS) signal, whereas the second probe laser beam probes only such noise. By comparing the output signals of the first and the second photo detector, the noise may be isolated from the desired PTPS signal. Thus, the different noise contributions may be eliminated or at least greatly reduced with a simple, reliable set-up that does not affect the sensitivity of the measurements. It is a particular advantage of this two-beam set-up that the transmitted first probe laser beam and the transmitted second probe laser beam may be detected simultaneously.
In a preferable embodiment, the photothermal interferometry apparatus further comprises a subtractor for subtracting a second transmission signal corresponding to the transmitted second probe laser beam from a first transmission signal corresponding to the first transmitted probe laser beam.
In a preferable embodiment,
the first and the third mirror are formed by a first and a second section of a first mirror element,
the second and the fourth mirror are formed by a first and a second section of a second mirror element such that
the first and the second cavity extend continuously between the first and second mirror element.
In this embodiment, the first and the second cavity extend continuously between the first and the second mirror element. The first probe laser beam intersects the excitation laser beam in the first cavity, whereas the second probe laser beam passes the excitation laser beam in the second cavity.
However, in an alternative embodiment, the first and the third mirror and the second and the fourth mirror, respectively, may be separate such that the first and the second cavity may extend separately. It is crucial, however, that the first and the second cavity contain the same sample in order to isolate the noise in the PTPS signal.
In a preferable embodiment, the first probe laser beam runs essentially perpendicularly to the excitation laser beam in the first cavity. In this embodiment, the first and the second probe laser beam may be guided in parallel through the first and the second cavity, respectively.
In an exemplary embodiment, one of the first and the second probe laser beam may be deflected for example by 90 degrees, in particular by means of reflection (for example with a mirror or prism), after emerging from the beam splitter.
In another exemplary embodiment, a beam splitting mirror is used as beam splitter for forming the first and the second probe laser beam with a lateral spacing from each other.
According to an aspect of the invention, a photothermal interferometry apparatus for detecting a molecule in a sample, in particular for detecting a trace gas species, comprises:
a Fabry-Perot interferometer with a first and a second mirror and a first cavity for containing the sample extending between the first and the second mirror,
a probe laser for passing a probe laser beam through the first cavity of the Fabry-Perot interferometer,
an excitation laser for passing an excitation laser beam through the first cavity of the Fabry-Perot interferometer for exciting the molecule in the sample,
a photodetector unit for detecting the transmitted probe laser beam passed through the first cavity of the Fabry-Perot interferometer.
A preferred embodiment comprises a beam splitter for splitting the probe laser beam into a first and a second probe laser beam and a first and a second photo detector for detecting the transmitted first probe laser beam intersecting the excitation laser beam and the transmitted second probe laser beam not intersecting the excitation laser beam, as described above.
However, some or all of following features may also be used in embodiments without such beam splitter, in particular in an arrangement with a single probe laser beam.
In a preferred embodiment, the photothermal interferometry apparatus further comprises
a modulator for modulating the wavelength of the excitation laser beam,
the photodetector unit being arranged for detecting a modulation of the transmitted probe laser beam passed through the first cavity of the Fabry-Perot interferometer.
In a preferred embodiment, the photodetector unit communicates with a control unit arranged for determining a harmonic, in particular a second harmonic, of the modulation of the probe laser beam passed through the first cavity of the Fabry-Perot interferometer. In this embodiment, the control unit comprises a demodulator for detecting a nth harmonic of the transmitted probe laser beam.
In a preferred embodiment, the control unit comprises a lock-in amplifier. In this embodiment, the lock-in amplifier serves as demodulator for detecting a nth harmonic of the transmitted probe laser beam.
Thus, PTS signal generation preferably is performed by periodic sample heating using modulated excitation radiation. Preferably modulation is accomplished by wavelength modulation (WM) where the emission frequency of the excitation laser is modulated. Wavelength modulation spectroscopy (WMS) is able to increase the signal to noise ratio (SNR) by reduction of the noise content of a measurement used for trace detection. By WM the absorption of the excitation laser beam is transformed into a periodic signal which preferably is isolated by a lock-in amplifier at its harmonics. This type of detection results in a significant improvement in the signal-to-noise ratio (SNR) by restriction of the detection pass band to a narrow frequency interval, as well as by shifting the detection to higher frequencies, where the 1/f laser noise is significantly reduced.
WM and second harmonic detection in particular (2f WM) offers the advantage that the detected signal is sensitive to spectral shape or curvature rather than absolute absorption levels. For example, by slowly tuning the center frequency over an absorption line, a spectrum which is roughly proportional to the second derivative may be obtained. Selectivity is furthermore increased by 2f detection because of efficient elimination of linear slops of spectra which greatly suppresses signals originating from broad featureless absorptions, such as undesired absorptions originating from the cell and its components, or pressure-broadened bands of large polyatomic molecules. These background absorptions are relatively flat in the observed wavelength region and thus only a tiny signal will be observed.
In a preferred embodiment, the photothermal interferometry apparatus further comprises a first tuner for tuning the probe laser beam over a first given wavelength range. This embodiment allows for fixing the probe laser wavelength via a feedback loop at around the inflection point of the transmission function of the FPI which is particularly favorable for obtaining good results in the measurements. The tuning of the probe laser may be done by adapting temperature and/or injection current, as is well known in the art.
In a preferred embodiment, the photothermal interferometry apparatus further comprises a second tuner for tuning the excitation laser beam over a second given wavelength range. The tuning of the excitation laser may be done by adapting temperature and/or injection current. Tuning of the excitation laser is particularly advantageous for the purpose of multi-analyte determinations.
For avoiding moveable components, the first and second mirror preferably are arranged immovably in a constant distance from each other. Thus, in this embodiment the first and second mirror are static and their relative arrangement need not be adjusted. This allows for a particularly stable set-up.
In a preferred embodiment, the Fabry-Perot interferometer comprises a sample cell for containing the sample, the first and the second mirror being fixed on a first and second side of the sample cell. In this way, the first and the second mirror are arranged in a constant distance from each other on opposite sides of the sample cell. This provides for a very stable arrangement suitable for mobile use.
In a preferable embodiment, an entry and an exit window for the excitation laser beam preferably are arranged opposite one another on the sample cell. The entry window may be arranged on a third side of the sample cell, whereas the exit window is arranged on a fourth side of the sample cell. In this way, the excitation laser beam may intersect the probe laser beam essentially perpendicularly in the first cavity.
In another embodiment, the probe laser beam may be collinear with the excitation laser beam in the first cavity inside the sample cell. In this embodiment, the excitation laser beam may be passed into and out of the sample cell through the first and second mirror of the Fabry-Perot interferometer.
In a preferred embodiment, the sample cell of the Fabry-Perot interferometer comprises a sample inlet and a sample outlet. The sample may be introduced to the first cavity in the sample cell through the sample inlet. After interaction with the excitation laser beam the sample leaves the sample cell through the sample outlet. In one preferred embodiment, the sample inlet is separate from the sample outlet. This embodiment is particularly suitable for actively passing a sample into the gas cell through the gas inlet and withdrawing the sample from the gas cell through the gas outlet. In another preferred embodiment, the sample inlet and the sample outlet are formed by a single opening which allows for diffusion of a sample into the sample cell.
In a preferred embodiment, the sample cell is a gas cell for containing a sample gas. However, the technology described herein is also suitable for investigation of liquid samples.
In a preferred embodiment, the photothermal interferometry apparatus further comprises a vacuum device connected to the sample outlet of the Fabry-Perot interferometer. The vacuum device is arranged for lowering the pressure inside the sample cell to a level below atmospheric pressure. The line shape of a molecular absorption depends on the sample gas pressure. At atmospheric pressure line shapes are broadened due to molecular collisions. As the sample pressure is reduced by means of the vacuum device the pressure broadened linewidth decreases preferably until thermal motion broadening dominates which will increase line shape curvature and thus sensitivity. Also selectivity will be greatly improved when the target absorption lines are resolved from interferents in a multi-gas sample such as water vapor.
In a preferred embodiment, the photothermal interferometry apparatus further comprises
a reference cell containing the sample, the reference cell being arranged, in the path of the excitation laser beam such that the excitation laser beam is passed through the sample in the reference cell,
a photo diode for detecting the excitation laser beam passed through the reference cell.
The photo diode preferably is connected to a further lock-in amplifier for demodulating an odd harmonic, preferable the third harmonic, of the transmitted excitation laser beam.
In this way, the accuracy of the measurements is further increased. In particular, the excitation laser beam may be fixed by a feedback loop to an absorption line of the sample such that a drift in the measurement may be avoided. Furthermore, the data acquisition time may be reduced. Also, the sensitivity may be improved.
In a preferred embodiment, the excitation laser is a diode laser, preferably a continuous wave quantum cascade laser, in particular a continuous wave distributed feedback quantum cascade laser, or an external cavity quantum cascade laser or an interband cascade laser, and/or wherein the probe laser is a diode laser, preferably a single mode diode laser, for example a continuous wave distributed feedback diode laser or external cavity quantum cascade laser. In this embodiment, the wavelength of the excitation laser and/or the wavelength of the probe laser may be tunable. Using a diode laser as excitation laser allows for wavelength modulation of the excitation laser beam by means of current tuning, which is particularly favorable in that no moveable components are required for this purpose. This yields a particularly stable apparatus.
According to another aspect of the invention, a method for detecting a molecule, in particular a trace gas species, in a sample using photothermal spectroscopy, comprises the steps of:
providing a probe laser beam,
directing the probe laser beam through the sample in a first cavity of a Fabry-Perot interferometer,
providing an excitation laser beam for heating the sample in the first cavity of the Fabry-Perot interferometer,
directing the excitation laser beam through the first cavity of the Fabry-Perot interferometer,
detecting the transmitted probe laser beam passed through the first cavity of the Fabry-Perot interferometer.
According to another aspect of the invention, a method for detecting a molecule, in particular a trace gas species, in a sample using photothermal spectroscopy, comprises the steps of:
providing a first and a second probe laser beam, preferably by splitting a probe laser beam into a first and a second probe laser beam,
directing the first probe laser beam through the sample in a first cavity of a Fabry-Perot interferometer,
directing the second probe laser beam through the sample in a second cavity of the Fabry-Perot interferometer,
providing an excitation laser beam for heating the sample in the first cavity of the Fabry-Perot interferometer,
directing the excitation laser beam through the sample in the first cavity of the Fabry-Perot interferometer,
detecting the transmitted first probe laser beam,
detecting the transmitted second probe laser beam.
The method preferably further comprises the step of subtracting a second transmission signal corresponding to the transmitted second probe laser beam from a first transmission signal corresponding to the transmitted first probe laser beam.
In a preferred embodiment, the method further comprises the steps of
detecting a thermal wave in the sample with the transmitted first probe laser beam (8a) and
detecting an acoustic wave in the sample with the transmitted second probe laser beam (8b).
Thus, the dual-beam arrangement described above is arranged for independently measuring the thermal wave and the acoustic wave induced by the interaction of the sample with the excitation laser beam in the first cavity. The thermal wave is observed through the first probe laser beam. The acoustic wave travels from the first cavity to the second cavity (which may be formed continuous with the first cavity) and thus influences the sample contained in the second cavity. The acoustic wave in the second cavity is observed through the second probe laser beam. The thermal wave and the acoustic wave have different properties. The thermal wave undergoes stronger attenuation having a wavelength below 1 mm. For this reason, the thermal wave may only be observed in the first cavity which is defined by the interaction with the excitation laser beam. The acoustic wave shows less attenuation having a wavelength of above 1 cm. This set-up improves the limit of detection of the molecule of interest as changes in the refractive index of opposite signs result from the thermal and acoustic wave, respectively. The temperature increase for the thermal wave leads to a decrease in the density of the sample, whereas the compression wave (acoustic wave) results in an increase of the density which influences the refractive index of the sample.
According to another aspect of the invention, a method for detecting a molecule, in particular a trace gas species, in a sample using photothermal spectroscopy, comprises the steps of:
providing a probe laser beam,
directing the probe laser beam through the sample in a first cavity of a Fabry-Perot interferometer,
providing an excitation laser beam for heating the sample inside the first cavity of the Fabry-Perot interferometer,
modulating the excitation laser beam wavelength,
directing the modulated excitation laser beam through the sample in the first cavity of the Fabry-Perot interferometer,
detecting a harmonic, in particular a second harmonic, of a modulation of the transmitted probe laser beam passed through the first cavity of the Fabry-Perot interferometer.
According to another aspect of the invention, a method for detecting a molecule, in particular a trace gas species, in a sample using photothermal spectroscopy, comprises:
providing a probe laser beam that can be tuned over a given wavelength range,
directing the probe laser beam through the sample in a first cavity of a Fabry-Perot interferometer,
tuning the probe laser beam in accordance with a predetermined value of a transmission function of the Fabry-Perot interferometer,
providing an excitation laser beam for heating the sample in the first cavity of the Fabry-Perot interferometer,
directing the excitation laser beam through the first cavity of the Fabry-Perot interferometer,
detecting the transmitted probe laser beam passed through the first cavity of the Fabry-Perot interferometer.
This embodiment is particularly favorable when investigating varying sample compositions.
Preferably, the predetermined value of the transmission function of the FPI corresponds essentially to the inflection point of the transmission function of the FPI, which may be at essentially 75% intensity transmission through the FPI. In this embodiment, the probe laser beam is tuned such that the intensity of the transmitted probe laser beam corresponds to the predetermined value (given in percent of the intensity of the probe laser beam as emitted by the probe laser).
In a preferable embodiment, a lock-in amplifier is arranged for receiving an AC (alternating current) signal from the photo detector unit, whereas a DC (direct current) signal from the photo detector unit may be used for maintaining the emission frequency of the probe laser at the predetermined value, preferably essentially at the inflection point, of the transmission function of the Fabry-Perot interferometer.